I have been told that this was illegal under the law that
No man shall submit to another
However, this does not sound to me like it is banning homosexuals, perhaps banning one from being a submissive but that would be all.
Is there any evidence that this activity was any rarer in Sparta than any other Greek state?
Edited in OP's comment: I was told it was one of Lycurgus's laws. However, I cannot find a reference to it anywhere on line